Variables & Constants in Node.js

Understand how variables and constants work in JavaScript: declaration, scope, hoisting, mutability, reassignment, and best practices.

Introduction to Variables

Variables are named containers used to store data so it can be reused and manipulated throughout a program.

1. What is a Variable?

Storing values in memory.

Declaring Variables

JavaScript provides three keywords to declare variables, each with different behavior.

1. var (Legacy)

Function-scoped declaration.

2. let

Block-scoped variable.

3. const

Block-scoped constant.

Mutability vs Reassignment

const prevents reassignment, not mutation.

1. Mutating const Objects

Allowed behavior.

Hoisting

Hoisting means JavaScript processes declarations before execution.

1. var Hoisting

Hoisted and initialized as undefined.

2. let / const Hoisting (TDZ)

Temporal Dead Zone.

Variable Scope

Scope determines the visibility and lifetime of a variable.

1. Block Scope

let and const.

2. Function Scope

var behavior.

Redeclaration & Reassignment

Different keywords allow different behaviors.

1. Redeclaration Rules

var vs let/const.

2. Reassignment Rules

const cannot be reassigned.

Best Practices

Following best practices avoids bugs and improves readability.

1. Recommended Guidelines

Industry standards.